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You know that sharp odor of chlorine from the swimming pool you
can recall from earliest childhood? It turns out it's not just
chlorine, but a potent brew of chemicals that form when chlorine
meets sweat, body oils, and urine.

But up until now, just how much urine has been difficult to
measure, says chemist Xing-Fang Li of the University of Alberta.
Li and her colleagues report they can now tell roughly how much
pee is in a pool by measuring the artificial sweeteners carried
in most people's urine. Certain sweeteners can be a good proxy
for pee, she says, because they're designed to "go right through
you" and don't break down readily in pool water.

The scientists calculated that one 220,000-gallon, commercial-
size swimming pool contained almost 20 gallons of urine. In a
residential pool (20-by-40-foot, five-feet deep), that would
translate to about two gallons of pee. It's only about one-
hundredth of a percent, but any urine in a swimming pool can be
a health concern for some people, not to mention that smell that
never quite goes away.

Li's team collected water from pools and hot tubs at hotels and
recreation facilities in two Canadian cities and measured the
amount of a sweetener called acesulfame potassium, or Ace-K, for
short. It's found in everything from yogurt to soup these days,
so it's no surprise that it's ubiquitous in our urine.

It's not just in the North American diet. Ace-K has been found
in people's urine in China too. And enough people are peeing in
pools for sweeteners to show up there, too.

"I think you can assume that if people are using your pool,
they're peeing in it," says Ernest Blatchley III, an
environmental engineer at Purdue University.

Apart from being gross, that's also a potential health hazard.
Chlorine reacts with urine to form a host of potentially toxic
compounds called disinfection byproducts. These can include
anything from the chloramines that give well-used pools the
aforementioned odor, to cyanogen chloride, which is classified
as a chemical warfare agent. There are also nitrosamines, which
can cause cancer. There's not enough evidence to say whether the
nitrosamine levels in pools increase cancer risk, Blatchley
says, but one study in Spain did find more bladder cancers in
some long-term swimmers.

That's not to say that people should avoid swimming, says
Clifford Weisel, an environmental health expert at Rutgers
University. Even people with respiratory problems like asthma
can benefit from the exercise, but "if you take your child to
the pool and they react to it, make sure you understand why," he
says. Indoor pools can be more problematic for people with
respiratory conditions, for example, because compounds build up
in the air above the water, and there's less natural sunlight,
which breaks down some harmful compounds.

The simplest solution: Just don't pee in the pool. And tell all
your friends not to do it, either. "I view it like secondhand
smoke," Blatchley says. "It's disrespectful and potentially
dangerous."

Also, swimmers should shower before getting in the pool, and get
out to go to the bathroom, Li says. Even a one-minute rinse
before diving in can remove much of the sweat and body gunk that
reacts with chlorine.

Once someone does pee in the pool, the only way to truly get rid
of it is to replace the water. "It's not uncommon for water in a
pool to go unchanged for years," Blatchley says, since many pool
owners or operators just add water as needed rather than
completely replacing it, which is more expensive.

And the longer water sits in a pool, the worse it gets, his
research has shown. Over time, people add more chlorine to the
water, which is converted to a form called chloride that builds
up and encourages the formation of yet more disinfection
byproducts.

Li says she's a regular swimmer, and doesn't want to discourage
people from a healthy activity. "This isn't to scare people,"
she says, "but hopefully they can prevent the problem."

The study was published Wednesday in Environmental Science &
Technology Letters.

You know that sharp odor of chlorine from the swimming pool you
can recall from earliest childhood? It turns out it's not just
chlorine, but a potent brew of chemicals that form when chlorine
meets sweat, body oils, and urine.

But up until now, just how much urine has been difficult to
measure, says chemist Xing-Fang Li of the University of Alberta.
Li and her colleagues report they can now tell roughly how much
pee is in a pool by measuring the artificial sweeteners carried
in most people's urine. Certain sweeteners can be a good proxy
for pee, she says, because they're designed to "go right through
you" and don't break down readily in pool water.

The scientists calculated that one 220,000-gallon, commercial-
size swimming pool contained almost 20 gallons of urine. In a
residential pool (20-by-40-foot, five-feet deep), that would
translate to about two gallons of pee. It's only about one-
hundredth of a percent, but any urine in a swimming pool can be
a health concern for some people, not to mention that smell that
never quite goes away.

Li's team collected water from pools and hot tubs at hotels and
recreation facilities in two Canadian cities and measured the
amount of a sweetener called acesulfame potassium, or Ace-K, for
short. It's found in everything from yogurt to soup these days,
so it's no surprise that it's ubiquitous in our urine.

It's not just in the North American diet. Ace-K has been found
in people's urine in China too. And enough people are peeing in
pools for sweeteners to show up there, too.

"I think you can assume that if people are using your pool,
they're peeing in it," says Ernest Blatchley III, an
environmental engineer at Purdue University.

Apart from being gross, that's also a potential health hazard.
Chlorine reacts with urine to form a host of potentially toxic
compounds called disinfection byproducts. These can include
anything from the chloramines that give well-used pools the
aforementioned odor, to cyanogen chloride, which is classified
as a chemical warfare agent. There are also nitrosamines, which
can cause cancer. There's not enough evidence to say whether the
nitrosamine levels in pools increase cancer risk, Blatchley
says, but one study in Spain did find more bladder cancers in
some long-term swimmers.

That's not to say that people should avoid swimming, says
Clifford Weisel, an environmental health expert at Rutgers
University. Even people with respiratory problems like asthma
can benefit from the exercise, but "if you take your child to
the pool and they react to it, make sure you understand why," he
says. Indoor pools can be more problematic for people with
respiratory conditions, for example, because compounds build up
in the air above the water, and there's less natural sunlight,
which breaks down some harmful compounds.

The simplest solution: Just don't pee in the pool. And tell all
your friends not to do it, either. "I view it like secondhand
smoke," Blatchley says. "It's disrespectful and potentially
dangerous."

Also, swimmers should shower before getting in the pool, and get
out to go to the bathroom, Li says. Even a one-minute rinse
before diving in can remove much of the sweat and body gunk that
reacts with chlorine.

Once someone does pee in the pool, the only way to truly get rid
of it is to replace the water. "It's not uncommon for water in a
pool to go unchanged for years," Blatchley says, since many pool
owners or operators just add water as needed rather than
completely replacing it, which is more expensive.

And the longer water sits in a pool, the worse it gets, his
research has shown. Over time, people add more chlorine to the
water, which is converted to a form called chloride that builds
up and encourages the formation of yet more disinfection
byproducts.

Li says she's a regular swimmer, and doesn't want to discourage
people from a healthy activity. "This isn't to scare people,"
she says, "but hopefully they can prevent the problem."

The study was published Wednesday in Environmental Science &
Technology Letters.